1. Field of the Invention
The present invention relates to polymer polyols, to methods of making poller polyols, and to polyurethanes made from such polymer polyols.
2. Background
Polymer polyols commonly are used to produce polyurethane foams. Basically, polymer polyols are produced by polymerizing one or more ethylenically unsaturated monomers dissolved or dispersed in a polyol in the presence of a free radical catalyst to form a stable dispersion of polymer particles in the polyol. Polymer polyols are valuable because they can produce polyurethane foam which has high load-bearing properties.
The first commercially accepted polymer polyols primarily were produced using acrylonitrile monomer, and had a somewhat higher viscosity than desired for some applications. More recently, polymer polyols of lower viscosity have been produced using acrylonitrile-styrene monomer mixtures.
Polyurethane foams made from polymer polyols have a wide variety of uses. The two major types of polyurethane foam generally are slabstock and molded foam. Slabstock foam made using polymer polyols typically is used in the carpet, furniture, and bedding industries. The primary type of molded foam, generally termed high resiliency (HR) molded foam, is used widely in the automotive industry for applications ranging from molded seats to energy-absorbing padding and the like.
The wide demand for polyurethane foams has spawned a need for polymer polyols that can produce foams having a wide variety of characteristics. For example, a demand exists for slabstock foam that is virtually scorch-free. It also is desirable for these scorch-free foams to have low density (viz.--1.5 pounds per cubic foot or less) while maintaining satisfactory load-bearing and other foam properties. One way to produce such a foam is to use a monomer mixture having a high styrene content (e.g., about 65 to 70 percent styrene).
The preparation of polymer polyols using a monomer mixture with a high styrene content creates difficulties. For example, the commercial processability of a particular polymer polyol depends upon its stability against phase separation, or its stability against the polymer particles settling out of the polyol medium. Many applications require rigorous stability, which becomes more difficult to achieve when high styrene content monomer mixtures are employed. It has been found that a higher stability polymer polyol may be obtained if the components used to make the polymer polyol are not fed to the reactor all at once. For example, U.S. Pat. No. 4,148,840 to Shah attempts to improve the stability of a polymer polyol by adding only a minor portion of a preformed polymer polyol to the base polyol along with the monomers and initiators. Another approach is seen in U.S. Pat. No. 4,242,249 to Van Cleve, et al., which is directed to the polymerization of an unsaturated macromonomer with other monomers to form a non-aqueous dispersion stabilizer which may be used in small amounts, 5% or less, to stabilize a polymer dispersion.
Other polyurethane foams that are in demand are foams that have high load-bearing capacities. A high load bearing capacity is particularly desirable in the slabstock area. The load-bearing capacity of a foam may be increased by increasing the polymer or solids content of the polymer polyol; however, as the solids content of the polymer polyol increases, the stability of the polymer polyol tends to decrease.
The trend toward the use of polymer polyols having a high styrene monomer mixture and a high solids content has resulted in polymer polyols that sometimes have a higher viscosity than desired. The viscosity of a polymer polyol must be low enough for ease in handling during manufacture and transport. At the same time, the stability of the polymer polyol must be high enough for use in the sophisticated, high-speed, large-volume equipment, machines, and systems now used to handle, mix, and react polyurethane-forming ingredients. Most importantly, the particles in the polymer polyol must be small enough to avoid plugging the filters, pumps, etc., used in such equipment.
Two basic types of processes have been used to produce polymer polyols--continuous processes and semi-batch processes. In a continuous process, the monomers, polyols, and initiator(s) typically are fed continously to a back mixed, stirred reactor in a manner that minimizes the monomer to polyol ratio. A continuous process tends to minimize settling of the vinyl polymer, and can produce a wide range of polymer polyols with acceptable dispersion stability.
In a semi-batch process, the vinyl monomers are fed slowly to a partially charged, agitated reactor to avoid excess free monomer concentration at any time during the polymerization. A semi-batch process is more difficult to control than a continuous process, which can achieve a steady state after line-out.
An example of a semi-batch process is found in European Patent No. 0 365 986, in which a semi-batch process is used to form graft copolymer dispersions. In order to form the graft copolymer dispersion, a graft polyol having 30% or less solids content is formed in a continuous process. The graft polyol product then is used as seed in the semi-batch process to produce graft polyols having 30% or more solids content and having a broad particle size distribution.
Even with the advanced state of the art in polymer polyol technology, there is a need for further improvement of polymer polyols to enhance their dispersion stability, to minimize their viscosity at higher solids levels, and to minimize the particle size of the polymers in the polyol.